Which religion Zoroastrianism or Judaism is the oldest?

Answer 1

Historically, Judaism became the descendant religion of the early Hebrew religion, which was founded on the Canaanite religion of the ancestors of the Hebrews.

Keel and Uehlinger (Gods, Goddesses and Images of God in Ancient Israel) say there were no strict boundaries between the Israelite/Judahite religion and that of its neighbours at the end of the ninth century and during the eighth century. At this stage, YHWH entered the Hebrew religion and was soon adopted as the national God, while maintaining a polytheistic pantheon, and so yhe Hebrew religion began diverging from its roots.

The religion of Judah was becoming monolatrous in the late monarchy, or from about the eighth century BCE, and this could be regarded as the beginning of a truly ethnic religion that we might now call Judaism. Monotheism was to arrive around the time of the Babylonian Exile.

Zoroastrianism is considered to have begun in southern Russia around 1700 BCE, before migrating to eastern Iran and then eventually to western Iran, where it would become the state religion of the Persian Empire.

Answer 2

The first continuous monotheistic tradition and religion as we know it, was and is Judaism, the tradition founded by Abraham. It began 3800 years ago, and its founding principle is that God is One.

Zoroastrianism, which might be suggested as another early contender, is not such a clear matter. There is no consensus on when Zoroaster lived.

Some might claim that monotheistic Judaism started later, as (for example) King Josiah made reforms in the late First Temple period. However, this is the view of secular academe, and flies in the face of tradition. Judaism had already been monotheistic for eight centuries by the time of Josiah. All he did was to eradicate the traces of the idolatrous influences of those Jews who had strayed from their own religion. This had happened repeatedly (such as with Jehoshaphat [2 Chronicles 17:6], and Samuel before him [1 Samuel 7:3-4]); and those who strayed into pagan practices never encompassed the entire people.